Harvard Psychological Studies, Volume 1 eBook

of ten judgments was uniformly of one sign (four to
six being counted as half) and the second half of
the opposite sign. The percentages of cases in
which the series presented such a progression are
as follows: In diffused light, 7.6%; in darkness,
point of regard illuminated, 18.3%; in complete darkness,
26.1%. The element of constant error upon which
such progressions depend is the tendency of the eye
to come to rest under determinate mechanical conditions
of equilibrium of muscular strain.

The relation of the successive judgments of a series
to the reinstatement of specific eye-strains and to
the presence of an error of constant tendency becomes
clearer when the distribution of those series which
show progression is analyzed simultaneously with reference
to conditions of light and darkness and to binocular
and monocular vision respectively. Their quantitative
relations are presented in the following table:

TABLE IV.

Illumination. Per Cent. Showing
Progress. Binocular. Monocular.

In light. 7.6 % 50 % 50 %
In darkness. 18.3 34.2 65.8

Among judgments made in daylight those series which
present progression are equally distributed between
binocular and monocular vision. When, however,
the determinations are of a luminous point in an otherwise
dark field, the preponderance in monocular vision of
the tendency to a progression becomes pronounced.
That this is not a progressive rectification of the
judgment, is made evident by the distribution of the
directions of change in the several experimental conditions
shown in the following table:

When the visual field is illuminated the occurrence
of progression in binocular vision is accidental,
the percentages being equally distributed between
upward and downward directions. In monocular
vision, on the contrary, the movement is uniformly
upward and involves a progressive increase in error.
When the illuminated point is exposed in an otherwise
dark field the progression is preponderatingly downward
in binocular vision and upward in vision with the single
eye. The relation of these changes to phenomena
of convergence, and the tendency to upward rotation
in the eyeball has already been stated. There
is indicated, then, in these figures the complication
of the process of relocating the ideal horizon by
reference to the sense of general body position with
tendencies to reinstate simply the set of eye-muscle
strains which accompanied the preceding judgment, and
the progressive distortion of the latter by a factor
of constant error due to the mechanical conditions
of muscular equilibrium in the resting eye.